Implantable cardiac rhythm management devices are commonplace today for the treatment of chronic or recurring cardiac arrhythmias. For example, cardiac pacemakers are implantable devices that replace or supplement a heart's compromised ability to pace itself (i.e., bradycardia) due to chronotropic incompetence or a conduction system defect by delivering electrical pacing pulses to the heart. Implantable cardioverter/defibrillators (ICD's) are devices that deliver electrical energy to the heart in order to reverse excessively rapid heart rates (tachycardia) including life threatening cardiac arrhythmias such as ventricular fibrillation. Since some patients have conditions that necessitate pacing and also render them vulnerable to life-threatening arrhythmias, implantable cardiac devices have been developed that combine both functions in a single device.
Cardiac rhythm management devices are typically implanted subcutaneously or submuscularly in a patient's chest and have leads threaded intravenously into the heart to connect the device to electrodes used for sensing and pacing. Leads may also be positioned on the epicardium by various means. A programmable electronic controller causes shocks to be delivered when fibrillation is detected or pacing pulses to be output in response to lapsed time intervals and sensed electrical activity. Pacemakers sense intrinsic cardiac electrical activity by means of internal electrodes disposed near the chamber to be sensed.
Modern cardiac rhythm management devices also typically have the capability to communicate data via a data link with an external programming device. Such data is transmitted to the pacemaker in order to program its mode of operation as well as define other operating parameters. Data transmitted from the pacemaker can be used to verify the operating parameters as well as relay information regarding the condition of both the pacemaker and the patient. Pacemaker patients are monitored at regular intervals as part of routine patient care and to check the condition of the device. Among the data which may typically be telemetered from the pacemaker are its programming parameters and an electrogram representing the electrical activity of the heart as sensed by the pacemaker.
Pacemakers have also been developed which monitor a patient's exertion level while the device is functioning in order to adjust the pacing rate. Such devices, referred to as rate-adaptive pacemakers, may use various measurable physiological parameters for this purpose that are related to exertion level including minute ventilation, body activity, electrogram intervals, and body temperature. Because of their continuous access to the patient and their communications capabilities, cardiac rhythm management devices and other similar implantable devices may also offer an ideal platform for gathering and storing clinically useful information which can later be transmitted to an external device.